Class II major histocompatibility complex (MHC) genes encode a family of cell surface heterodimers that function in the initiation of the immune response by presenting antigens to T lymphocytes. The expression of class II MHC gene products is tightly regulated as abnormal patterns or expression leads to immune dysfunction. This is clearly seen in an autosomal recessive disorder, the class II deficient congenital immunodeficiency (CID) or bare lymphocyte syndrome, where class II expression is lost due to a defect in the ability to transcribe class II genes. The inappropriate expression of class II MHC molecules in cells that normally do not express them has long been postulated as mechanism that leads to the onset of many autoimmune diseases. Therefore, elucidating the molecular mechanisms that regulate class II gene expression is important in understanding how the immune response is controlled and how alterations in expression may ultimately influence the progression of these diseases. this grant will focus on the isolation and characterizing of cDNA clones that regulate the expression of the class II major histocompatibility complex (MHC) genes. The approach used will be to functionally complement the defects in two class II transcription-negative mutant-cell lines, RJ2.2.5 and SJO. The mutations in these cell lines are due to global transcriptional regulators of class II MHC gene expression. Stable transfectants of these cell lines will be made that contain a neomycin gene under the control of a class II MHC gene promoter. These cell lines will then be transfected with an episome-based cDNA expression library to activate the class II promoter. Since this promoter is not functional in these cell lines, the ability of these cells to become neomycin resistance will depend upon functional complementation of the regulatory gene defect. Rescue of the complementing cDNA, molecular and biochemical characterization of the clones and their encoded products will be carried out to elucidate the mechanisms of class II gene regulation. Understanding the nature of the regulatory genes may allow the future development of reagents and protocols to control class II mediated immune responses in clinical settings, such as organ transplantation or in autoimmune disease.